Process for the separation of fatty acids from reaction mixtures producing same



United States PROCESS FOR THE SEPARATION OF FATTY ACIDS FROM REACTION IVIIXTURES PRO- DUCIN G SAlVlE Shirley P. Lingo, Terre Haute, Ind., assiguor to Commercial Solvents Corporation, Terre Haute, Ind., a corporation of Maryland No Drawing. Application January 2, 1953,

Serial No. 329,480

4 Claims. (Cl. 260-540) My invention relates to the process for separating lower fatty acids and nitroalkanes from mixtures comprising a lower fatty acid, nitroalkanes, and an inorganic acid. More particularly my invention relates to a process for separating lower fatty acids in. substantially pure form and nitroalkanes substantially free of acidic impurities from reaction mixtures containing them.

Mixtures containing a lower fatty acid, nitroalkanes and an inorganic acid can be produced according to the procedure set out in United States Patent No. 2,113,812, issued to Samuel B. Lippincott on April 12, 1938, as well as by other methods. The method of Lippincott set out in the above cited U. S. patent results in a mixed product in the form of two distinct layers. The lower layer contains hydroxylamine in the form of a mineral acid salt and most of the unreacted mineral acid employed in the reaction. The upper layer contains the fatty acid produced in the reaction, unreacted nitroalkanes, the remaining portion of the unreacted mineral acid employed and traces of the hydroxylammonium salt of the mineral acid. When this upper layer is decanted from the lower the hydroxylammonium salt of the mineral acid contained in the lower layer can be employed in other processes in that form. The mixture from which substantially pure lower fatty acids and nitroalkanes substantially free of acidic impurities are separated by my process consists of the decanted upper layer of the preferred ,Lippincott process reaction product.

The object of my invention is a process for the separation of substantially pure lower fatty acids containing from two to six carbon atoms per molecule from a mixture containing the fatty acids, nitroalkanes and a mineral acid. Another object of my invention is a process whereby the nitroalkanes contained in the initial mixture may be recovered substantially free of acidic impurities. Another object of my invention is a method of separation which permits the recovery of substantially pure lower fatty acid from the initial mixture without the losses of the fatty acid which occur in recovery processes previously available.

It has been the practice to wash the initial mixture containing fatty acid, unreacted nitroalkanes and unreacted mineral acid with an amount of water equivalent to 6 to 12% by volume of the mixture being treated and then treat the washed mixture with suificient concentrated sodium hydroxide solution to neutralize the remaining unreacted mineral acid. Thereafter, the washed mixture was subjected to distillation to remove the unreacted nitroalkanes and recover the fatty acid. .This practice has many drawbacks, among the worst of which is that the fatty acid recovered is of insufficient purity for use in many processes. Other disadvantages include the large loss of fatty acid. in the water washing step, the caking and insulating of the heat transfer coils of the distillation vessels because of the solidified sodium salts deposited from solution, and the production of a final product containing sufficient-amounts of unreacted nitroalkanes and decomposition products of reducing activity Patent O "ice to render the product too impure for use in many processes. The unreacted nitroalkanes recovered from the distillation step were likewise contaminated with fatty acid and this portion of the fatty acid represented an unrecoverable loss of the desired product. The treatment with caustic solution was responsible for at least two of the difliculties experienced. The deposition of sodium salts on the heating coils of the distillation vessel greatly complicated the distillation procedure. It necessitated the use of higher coil temperatures and the continuous increase of these temperatures to overcome the insulating elfect of the caked salts. Therefore, poor temperature control was maintained in the distillation step, and frequent costly shutdowns for cleaning were also necessitated. The composition of the mixture undergoing distillation, coupled with the lack of temperature control which frequently allowed temperatures well above the boiling point of the fatty acid product, resulted in the formation of undesirable decomposition products of reducing activity which were carried over into the final product. i

I have now discovered that a substantially pure lower fatty acid containing from two to six carbon atoms per molecule and nitroalkanes substantially free of acidic impurities may be separated from an initial mixture of lower fatty acid, nitroalkanes, and unreacted mineral acid. My new process for the separation comprises subjecting the said mixture of a lower fatty acid, nitroalkanes, and unreacted mineral acid to washing with from about 5% to about 7.5 by volume of water, contracting the resulting water-washed mixture with an anion exchange material, subjecting the effluent from the said anion exchange material to distillation, contacting the nitroalkane fraction separated with an anion exchange material in alkaline form and recovering the substantially pure lower fatty acid and nitroalkanes substantially free of acidic impurities.

In the first step of my process a mixture containing a fatty acid, nitroalkanes, and an inorganic acid; such as the mixture decanted from the lower layer of the product of the Lippincott process, is washed with from about 5% to about 7.5% by volume of water to remove as much as possible of the unreacted mineral acid while still minimizing loss of desired fatty acid. I have found that wash waters held between these limits will reduce the concentration of the mineral acid anion from about 7% to about 0.4% while at the same time removing generally only from about 2%-to about 3.5% of the total fatty acid content.

The resulting water-washed, but still essentially anhydrous, mixture is next contacted with an anion exchange material to complete the removal of the mineral acid anion. This operation may be carried out readily in a suitably designed column. Anion exchange materials suitable for use in my process include the basic anion exchange materials having amine, mixed amine, or quaternary ammonium polar groups. Typical of exchange materials having mixed amine polar groups are Amberlite IR-45, a trade-marked product of Rohm & Haas, Inc., and Permutit A, a trade-marked product of The Permutit Co. Typical of the group of suitable exchange materials containing quaternary ammonium polar groups are Amberlite IRA 400, and Amberlite IRA 410, trade-marked products of Rohm & Haas, Inc., and Permutit Sl, a trade-marked product of The Permutit Co. It has been found that any of the anion exchange materials containing these types of polar groups will reduce the concentration of mineral acid anion contained in a mixture of lower fatty acid and nitroalkanes from a range of about .3% to 2% to a final concentration of approximately 40 parts per million or less.

3 process can be contacted directly in their basic form with the crude water washed mixture of fatty acid and nitroalkanes, or they can be first converted to the form of the salt of the fatty acid to be recovered. This latter procedure is preferred in that it avoids any possibility of overheating the anion exchange material and the consequent destruction of a part of its exchange activity by the heat of neutralization generated by contacting the alkaline form of the anion exchange material with a mineral acid. The conversion of the anion exchange materials to their fatty acid salt form is also desirable as a method of re covering otherwise difficulty recoverable or unrecoverable fatty acids. The exchange materials may be converted conveniently and at a saving in fatty acid by contacting them with any fraction from the subsequent fractional distillation step containing the fatty acid and distilling at a vapor temperature below the boiling point of the purified acid. One such fraction is the intermediate fraction consisting of water and a relatively small percentage of fatty acid. This recovery of the fatty acid realized by conversion of the anion exchange material would otherwise necessitate an additional distillation step or incorporation of the fraction with the next batch for distillation. Thus, treating the exchange material with this fraction avoids an additional recycling or distillation procedure.

Another potential source of fatty acid for conversion of the anion exchange material is the lowest boiling fraction from the subsequent distillation step, for if distillation conditions are inaccurately controlled some fatty acid will be included in the water-nitroalkane first fraction. This is a particularly valuable discovery since there has been no economical method of recovering the small percentage of fatty acid included in the lowest boiling fraction, and it has been regarded as unrecoverable loss when distillation was the only recovery procedure used. In such cases it has also comprised an impurity in the recovered nitroalkanes rendering the latter unfit for use in many other processes. The recovery by an anion exchange material of small amounts of fatty acids from the water-nitroalkane fraction thus represents a saving of otherwise unrecoverable fatty acids and permits the re covery of the nitroalkanes present substantially free of all acidic impurities.

The effluent from the anion exchange material treatment now substantially freed of mineral acid anions and still essentially water-free is next subjected to fractional distillation to separate the components of the efiiuent. For those mixtures the components of which possess boiling points too close for effective direct fractionation, and the nitroalkane component of which forms an azeotrope with water, an amount of water sufficient to form an azeotropic mixture with all the nitroalkane present is added, either before or during the course of the distillation. When such mixtures are distilled the lowest boiling fraction recovered consists of' an azeotropic mixture of the nitroalkanes present and a substantial portion of the water added to form the azeotrope. Of this fraction the nitroalkane layer which separates is decanted and then recycled to a fresh column of the anion exchange material in the alkaline form to recover any traces of the fatty acid contained therein and to yield a nitroalkanc mixture with water free of all acidic impurities. The second, or intermediate, fraction consists of the remainder of the water and a small portion of the fatty acid contained in the effluent. This fraction is then recycled to a column of the anion exchange material at least partially in the alkaline form to recover the fatty acid present as the fatty acid salt form of the exchange material. The third fraction consists of substantially pure fatty acid. Careful attention to distillation temperatures and reflux ratios will retain any higher boiling products in the distillation residue which is subsequently discarded. The fractional distillation may be conducted in anydistillation equipment adapted to careful control of distillation temperatures and effective fractionation.

My invention may be utilized to separate any of the lower fatty acids and nitroalkanes which can be distilled in commercial fractionation equipment. These fatty acids include those alkyl carboxylic acids containing from two to six carbon atoms per molecule, such as acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid. The nitroalkanes which may be separated free of acidic impurities from these acids are those nitroalkanes containing the same or a lesser number of carbon atoms per molecule or the acids from which they are separated, such as nitroethane, nitropropanes, nitrobutanes, nitropentanes, and nitrohexanes.

The following examples are offered to illustrate my invention and it is understood that I do not intend to limit it to the exact amounts and procedures set forth therein, but I intend for all equivalents of my invention apparent to those skilled in the art to be specifically included within the scope of this specification and attached claim.

Example I Propionic acid was prepared by the method of Lippincott from 1,067 ml. of l-nitropropane, ml. of water, and 1,020 gm. 96% sulfuric acid. The reaction product consisting of two discrete layers was separated at 60 C. The upper layer consisting of 970 ml. had the following percentage composition: propionic acid 33.4%, l-nitropropane 41.2%, hydroxylammonium acid sulfate 5.6%, sulfuric acid 12.6%, and water 3.4%. A 940 ml. portion of the above mixed product was washed with 55 ml. of water with agitation and the two layers which resulted were separated. The lower aqueous layer containing 207 ml. was discarded. The upper washed crude mixture contained 790 ml. in which the following impurities remained: water 3.67%, hydroxylammonium acid sulfate 0.603%, and sulfuric acid 0.742%. A 760 ml. portion of the above water-washed, crude mixture was treated in a column with 50 ml. of Amberlite IR-400 anion exchange resin in the free base form. From this processing were recovered 740.6 gm. of washed, resintreated, crude mixture in which the following impurities were present: water 2.49%, hydroxylammonium acid sulfate 0.39%, and sulfuric acid 28 p. p. m. This mixture was then subjected to distillation and separation of the nitroalkane and fatty acid present as described below.

The distillation of a water washed resin treated crude mixture is illustrated by Examples II and III below.

Example 11 A Braun column of 22 plates operated at a 4:1 reflux ratio was employed to fractionally distill 500 ml. of a water-washed, resin-treated, crude'mixture consisting of l-nitropropane 23.8%, propionic acid 76.3%, hydroxylammonium acid sulfate 288 p. p. m., and sulfuric acid 14 p. p. m. This crude mixture had been washed with 60 ml. of water per 1,000 ml. of sample and the upper layer resulting from that wash had been treated in a column with 50 ml. of Amberlite IR-45 anion exchange resin in the free base form. This processing had resulted in the crude material with the above analysis. With the 500 ml. of washed, resin-treated, crude mixture was charged 38 ml. of water and during the course of the removal of the first fraction a total of 50 ml. of Water was recycled from the lower layer of the binary azeotrope which distilled. When distillation temperatures indicated that all the nitroalkane had been removed no further water was recycled. The first fraction consisting of l-nitropropane and water was removed in two cuts, ml. of product boiling from 91 to 92 C. from which the lower phase water was recycled. The upper phases of this first cut contained 95.1% l-nitropropane and 0.02% propionic acid. An additional 40 ml. of the first fraction was collected from a vapor temperature of 92 C. to 99 C. which contained 76.3% l-nitropropane,

20.7% water and 3.0% propionic acid. intermediate fraction was collected in the following two cuts: 55 ml. of mixture with a vapor pressure of from 99.5 C. to 100.5 C. consisting of 12.0% l-nitropropane, 70.2% water, and 17.5% propionic acid, and 20 ml. of a mixture with a vapor temperature of from 101 C. to 141 C. consisting of 4.5% l-nitropropane, 23.4% water, and 72.2% propionic acid. Purified propionic acid was collected in two cuts. The first, 65 ml. with a vapor temperature of 141 C., consisted of 99.95% propionic acid by weight; the second, which was distilled without reflux at a 1:1 ratio, contained 170 ml. consisting of 100.35% by weight propionic acid. Both samples of the propionic acid fraction contained only traces of reducing materials and were essentially pure.

Example 111 A packed distillation column of 25 inches packing height, equivalent to 17 theoretical plates, was employed to distill 300 ml. of a water-washed, resin-treated, crude propionic acid mixture. A reflux ratio of 1:1 was employed. The crude propionic acid mixture after washing and resin treating consisted of propionic acid 68.29%, l-nitropropane 27.15%, water 3.38%, sulfuric acid 28 p. p. m., and hydroxylammonium acid sulfate 257 p. p. m. A nitroalkane and water fraction of 50 ml. was collected from a vapor temperature of 88.5 C. to 91 C. The nitroalkane upper layer of this fraction contained only 0.37% propionic acid while the aqueous lower layer contained 1.57% propionic acid. An intermediate fraction of 70 ml. was collected at a vapor temperature of from 91 C. to 97 C. which likewise separated into two layers. The upper layer contained 5.76% propionic acid, 1.52% water, in addition to l-nitropropane, while the lower layer consisted of 90.77% water and 7.29% propionic acid. The propionic acid fraction of 150 ml. recovered at a vapor temperature of from 140 C. to 140.75" C. analyzed 100.14% by weight propionic acid and 0.135% water. This sample contained only traces of reducing substances and no detectable nitropropanes.

Example IV A crude mixture of acetic acid, nitroethane, and sulfuric acid was prepared from 375 gm. of glacial acetic acid, 120 gm. of nitroethane, and 5 gm. of sulfuric acid. This crude mixture containing one percent by weight of sulfuric acid as an impurity was then passed through a column of 50 ml. of Amberlite IRA-400 anion exchange resin in the form of the free base. This treatment resulted in a mixture containing less than 20 p. p. m. of sulfuric acid which mixture upon the addition of suflicient water can be separated readily by distillation into a nitroethane and water azeotrope and acetic acid.

Example V A crude mixture of butyric acid, l-nitrobutane, and sulfuric acid was prepared from 750 gm. of N-butyric acid, 240 gm. of l-nitrobutane, and gm. of sulfuric acid. This crude mixture containing one percent by weight of sulfuric acid as an impurity was then passed through a column of 50 ml. of Amberlite IR-45 anion exchange resin in the form of the butyrate salt. This treatment resulted in a mixture containing less than 20 p. p. m. of sulfuric acid which mixture upon the addition of suflicient water can be separated readily by distillation into a l-nitrobutane and water azeotrope and butryric acid.

Example VI A mixture was prepared consisting of 100 gm. of water, 300 gm. of l-nitropropane, and 10 gm. of propionic acid. The layers were separated mechanically. The lower or aqueous layer contained 3.63 gm. of propionic acid per 100 gm. of mixture; the upper or nitropropane layer contained 2.16 gm. of propionic acid per 100 gm. of

mixture-a total of 6.48 gm. These layers were then separately passed through a column of 50 ml. of Amberlite IR-400 anion exchange resin in the form of the free base. The efiluents of each layer were then analyzed for the presence of propionic acid. Careful potentiometric titration revealed no propionic acid present in either efiluent.

Now having described my invention what I claim is:

1. The process of separating a substantially pure fatty acid containing from two to six carbon atoms per molecule and nitroalkanes containing from two to six carbon atoms per molecule substantially free of acidic impurities" from a mixture comprising the fatty acid, nitroalkanes, and inorganic acid, which comprises subjecting the said mixture to washing with from about 5% to about 7.5% by volume of water, contacting the resulting water-washed mixture with an anion exchange material which has been converted into the form of a salt of the fatty acid being separated by contact with a fatty acid-containing fraction from the distillation of a previous batch of the said mixture from which the inorganic acid has been removed, subjecting the effluent from the said anion exchange material to distillation to remove a first nitroalkane fraction consisting essentially of a mixture of nitroalkane, water, and a trace of inorganic acid, a second nitroalkane fraction consisting essentially of a mixture of nitroalkane, water and fatty acid and a purified fatty acid fraction, contacting the second nitroalkane fraction separated in the distillation with an anion exchange material in alkaline form to recover nitroalkanes substantially free of acidic impurities and to convert the resin into a salt of the fatty acid for subsequent contact with a water-washed mixture of fatty acid, ntiroalkane and inorganic acid.

2. The process of separating substantially pure propionic acid and nitropropane substantially free of acidic impurities from a mixture comprising propionic acid, nitropropanes and inorganic acid, which comprises subjecting the said mixture to washing with water in the amount of from 5.5% to 7.5% by volume of the said mixture, contacting the resulting water-washed mixture with an anion exchange material which has been converted into the form of a salt of propionic acid by contact with a propionic acid-containing fraction from the distillation of a previous batch of the said mixture from which the inorganic acid has been removed, adding sufficient water to the water-washed mixture to form an azeotrope with all the nitropropane present, subjecting the effluent from the said anion exchange material and added water to distillation to remove a first nitropropane fraction consisting essentially of a mixture of nitropropane, water, and a trace of inorganic acid, a second nitropropane fraction consisting essentially of a mixture of nitropropane, water and propionic acid and a purified propionic acid fraction, contacting the second nitroalkane fraction separated in the distillation with an anion exchange material in alkaline form to recover nitropropane substantially free of acidic impurities and to convert the resin into a salt of propionic acid for subsequent contact with a water-washed mixture of propionic acid, nitropropane, and inorganic acid.

3. The process of separating substantially pure acetic acid and nitroethane substantially free of acidic impurities from a mixture comprising acetic acid, nitroethane and an inorganic acid, which comprises subjecting the said mixture to washing with water in the amount of from about 5% to about 7% by volume of the said mixture, contacting the resulting water-washed mixture with an anion exchange material which has been converted into a salt of acetic acid by contact with an acetic acid-containing fraction from the distillation of a previous batch of the said mixture from which the inorganic acid has been removed, adding sufficient water to the water-washed mixture to form an azeotrope with all the nitroethane present, subjecting the effluent from said anion exchange material and added water, to distillation to remove a nitroethane fraction consisting essentially of a mixture of nitroethane, water and a trace of acidic impurity, an intermediate fraction consisting essentially of a mixture of acetic acid and water and a purified acetic acid fraction, contacting the second nitroethane fraction separated in the distillation with an anion exchange mate rial in the alkaline form to recover nitroethane substantially free of acidic impurities and to convert the resin into a salt of acetic acid for subsequent contact with a water-washed mixture of acetic acid, nitrocthane and inorganic acid.

4-. The process of separating substantially pure butyric acid and nitrobutanes substantially free of acidic impurities from a mixture comprising butyric acid, nitrobutanes, and an inorganic acid, which comprises subjecting the said mixture to washing with water in the amount of from about 5.5% to about 7.5% by volume of the said mixture, contacting the resulting water-washed mixture with an anion exchange material which has been converted into the form of a salt of butyric acid by contact with a butyric acid-containing fraction from the distillation of a previous batch of the said mixture from which the inorganicacid has been removed, adding sufficient water to the water-Washed mixture to form an azeotrope with all the nitrobutane present, subjecting thc effluent from the said anion exchange material and added water to distillation to remove a nitrobutane fraction consisting essentially of nitrobutane, water and a trace of acidic impurity, an intermediate fraction consisting essentially of a mixture of butyric acid and water and a purified butyric acid fraction, contacting the second nitrobutane fraction separated in the distillation with an anion exchange material in the alkaline form to recover nitrobutanes substantially free of acidic impurities and to convert the resin into a salt of butyric acid for subsequent contact with a water-washed mixture of butyric acid, nitrobutane, and inorganic acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,113,812 Lippincott Apr. 12, 1938 2,341,907 Cheetha m et a1 Feb. 15, 1944 2,415,558 Hesler et al. Feb. 11, 1947 

1. THE PROCESS OF SEPARATING A SUBSTANTIALLY PURE FATTY ACID CONTAINING FROM TWO TO SIX CARBON ATOMS PER MOLCULE AND NITROALKANES CONTAINING FROM TWO TO SIX CARBON ATOMS PER MOLECULE SUBSTANTIALLY FREE OF ACIDIC IMPURITIES FORM A MIXTURE COMPRISING THE FATTY ACID, NITROALKANES, AND INORGANIC ACID, WHICH COMPRISES SUBJECTING THE SAID MIXTURE TO WASHING WITH FROM ABOUT 5% TO ABOUT 7.5% BY VOLUME OF WATER, CONTACTING THE RESULTING WATER-WASHED MIXTURE WITH AN ANION EXCHANGE MATERIAL WHICH HAS BEEN CONVERTED INTO THE FORM OF A SALT OF THE FATTY ACID BEING SEPARATED BY CONTACT WITH A FATTY ACID-CONTAINING FRACTION FROM THE DISTILLATION OF A PREVIOUS BATCH OF THE SAID MIXTURE FROM WHICH THE INORGANIC ACID HAS BEEN REMOVED SUBJECTING THE EFFLUENT FROM THE SAID ANION EXHANGE MATERIAL TO DISTILLATION TO REMOVE A FIRST NITROALKANE FRACTION CONSISTING ESSENTIALLY OF A MIXTURE OF NITROALKANE, WATER,AND A TRACE OF INORGANIC ACID, A SECOND NITROALKANE FRACTION CONSISTING ESSENTIALLY OF A MIXTURE OF NITROALKANE, WATER AND FATTY ACID AND A PURIFIED FATTY ACID FRACTION, CONTACTING THE SECOND NITROALKANE FRACTION SEPARATED IN THE DISTILLATION WITH AN ANION EXCHANGE MATERIAL IN ALKALINE FORM TO RECOVER NITROALKANES SUBSTANTIALLY FREE OF ACIDIC IMPURITIES AND TO CONVERT THE RESIN INTO A SALT OF THE FATTY ACID FOR SUBSEQUENT CONTACT WITH A WATER-WASHED MIXTURE OF FATTY ACID, NITROALKANE AND INORGANIC ACID. 